# Patent

## Title
Control circuit and bulk DC/DC converter in constant on-time mode

## Abstract

Constant on-time control circuit includes a comparing circuit including a comparator including a positive input end for receiving a control voltage; a negative input end for receiving a feedback voltage from the output voltage of the DC/DC converter; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the output end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal to control a switch set of the DC/DC converter according to the comparing signal.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bulk DC/DC converter, and more particularly, to a bulk DC/DC converter in constant on-time mode.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional bulk DC/DC converter 100. The bulk DC/DC converter 100 converts an input voltage source VIN to be an output voltage source VOUT, wherein the voltage VOUT is lower than the voltage VIN. As shown in FIG. 1, the DC/DC converter 100 comprises a control circuit 110, a switch set 120, an inductor L, an output capacitor COUT, and a voltage-dividing set 130. The switch set 120 comprises two switches Q1 and Q2. The voltage-dividing set 130 comprises two voltage-dividing resistors RB1 and RB2. The control circuit 110 comprises a comparator CMP1, a pulse generator 111, and a drive circuit 112. The operation principles of the bulk DC/DC converter 100 are described as follows.
The control circuit 110 controls the operation of the DC/DC converter 100 by constant on-time manner. That is, when the control circuit 110 detects the output voltage VOUT is lower than a predetermined value, the switch Q1 is turned on for a constant period of time (constant on-time) by the control circuit 110 (while the switch Q2 is turned off) for allowing the input voltage source VIN conducting to the inductor L through the switch set 120.
During the operation of the DC/DC converter 100, the inductor L carries current IL, and the current IL flows into the equivalent serial resistor RE of the output capacitor COUT so that the resistor RE carries voltage VL reflecting the current IL. As shown in FIG. 1, the waveform of the voltage VL is saw-toothed because the switch Q1 is periodically turned on/off. The comparator CMP1 receives the feedback voltage VFB divided from the voltage VL and the output voltage VOUT by the resistors RB1 and RB2, and compares with a reference voltage VREF1, so as to determine when to turn on the switch Q1. More specifically, when the voltage (feedback voltage VFB) on the negative input end of the comparator CMP1 is lower than the voltage (reference voltage VREF1) on the positive input end of the comparator CMP1, which means the output voltage VOUT is too low, and the switch Q1 is needed to be turned on for allowing the input voltage source VIN to charge the inductor L and the output capacitor COUT, the comparator CMP1 controls the pulse generator 111 to generate a pulse signal PON. When the pulse generator 111 is triggered by the comparator CMP1, the pulse generator 111 generates a pulse signal PON with a predetermined duration TP and predetermined logic. The drive circuit 112 controls the switch set 120 according to the pulse signal PON. More particularly, when the drive circuit 112 receives the pulse signal PON, the switch Q1 is driven to turn on for the predetermined duration TP. In addition, except in the dead time both of the switches Q1 and Q2 are turned off, when the switch Q1 is turned on, the switch Q2 is turned off; when the switch Q1 is turned off, the switch Q2 is turned on. In this way, the control circuit 110 controls the DC/DC converter 100 to operate regularly in constant on-time mode.
However, not all kinds of capacitors definitely have equivalent serial resistors, and because of the improvement to the manufacture of capacitors, the equivalent serial resistances of the capacitors become smaller, or even do not exist. For example, the multi-layer ceramic capacitor (MLCC) is very similar to an ideal capacitor and therefore the equivalent serial resistor does not exist on the MLCC. Consequently, when the MLCC is utilized as the output capacitor COUT, the resistor RE does not exist, and thus the information of the current IL cannot be informed to the control circuit 110, causing the control circuit 110 unable to control the DC/DC converter 100 according to the voltage VL effectively. The control circuit 110 is still able to operate by the feedback of the output voltage VOUT. However, the phase of the output voltage VOUT is far behind the phase of the voltage VL because of the output capacitor COUT, which makes the control circuit 110 unable to react to the variation of the output voltage VOUT in time. For this reason, the DC/DC converter 100 is not able to operate stably in constant on-time mode while utilizing the MLCC as the output capacitor, causing inconvenience.
SUMMARY OF THE INVENTION
The present invention provides a control circuit in constant on-time mode for controlling a DC/DC converter, comprising a comparing circuit comprising a comparator comprising a positive input end for receiving a control voltage; a negative input end for receiving a feedback voltage divided from an output voltage of the DC/DC converter; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal for controlling a switch set of the DC/DC converter.
The present invention further provides a bulk DC/DC converter in constant on-time mode, comprising a switch set coupled between an input voltage source and ground; an inductor coupled to the switch set for receiving the input voltage source through the switch set; an output end coupled to the inductor for outputting an output voltage; an output capacitor coupled between the capacitor and the ground; a voltage-dividing set coupled between the output end and the ground for generating a feedback voltage according to the output voltage; and a control circuit, comprising a comparing circuit comprising a comparator comprising a positive input end for receiving a control voltage; a negative input end coupled to the voltage-dividing set for receiving the feedback voltage; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; and a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal so as to transmit the input voltage source to the inductor through the switch set.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating a conventional bulk DC/DC converter.
FIG. 2 is a diagram illustrating a bulk DC/DC converter of the present invention.



DETAILED DESCRIPTION
Please refer to FIG. 2. FIG. 2 is a diagram illustrating a bulk DC/DC converter 200 of the present invention. The bulk DC/DC converter 200 converts an input voltage source VIN to be an output voltage source VOUT, wherein the voltage VOUT is lower than the voltage VIN. As shown in FIG. 2, the DC/DC converter 200 comprises a control circuit 210, a switch set 220, an inductor L, an output capacitor COUT, and a voltage-dividing set 230. The switch set 220 comprises two switches Q1 and Q2. The voltage-dividing set 230 comprises two voltage-dividing resistors RB1 and RB2. The control circuit 210 comprises a pulse generator 211, a drive circuit 212, a comparing circuit 213, and an amplifying circuit 214. The comparing circuit 213 comprises a comparator CMP2, and a voltage adjusting circuit 215. The voltage adjusting circuit 215 comprises a voltage reference source VREF2, a constant current source IS, a switch SW, a resistor RX, and a capacitor CX. The amplifying circuit 214 comprises an amplifier OP, a voltage reference source VREF3, a resistor R1, and a capacitor C1. The switch SW comprises a first end 1, a second end 2, and a control end C. It is noticeable that the output capacitor COUT utilized in the DC/DC converter 200 is assumed to not have the equivalent serial resistor. The operation principles of the bulk DC/DC converter 200 are described as follows.
The control circuit 210 controls the operation of the DC/DC converter 200 by constant on-time manner. That is, when the control circuit 210 detects the output voltage VOUT is lower than a predetermined value, the switch Q1 is turned on for a constant period of time (constant on-time) by the control circuit 210 (while the switch Q2 is turned off) for allowing the input voltage source VIN conducting to the inductor L through the switch set 220.
Because the output capacitor COUT used in the DC/DC converter 200 is assumed to have no equivalent serial resistor, the information about the current carried by the inductor L is not able to feed back to the control circuit 210. Thus, the control circuit 210 is designed to control the DC/DC converter 200 to operate regularly without the equivalent serial resistor.
In the amplifying circuit 214, the amplifier OP receives the feedback voltage VFB through the voltage-dividing set 230 dividing the output voltage VOUT, and the received voltage VFB is amplified through the resistor R1 and the capacitor C1 to be the amplified feedback voltage VFBO. In addition, the voltage reference source VREF3 provides a reference voltage VREF3.
In the comparing circuit 213, the comparator CMP2 compares the control voltage (VC) on its positive input end and the amplified feedback voltage VFBO on its negative input end, and accordingly outputs a comparing signal SC. The voltage reference source VREF2 provides a reference voltage VREF2. Thus, the control voltage VC equals to the reference voltage VREF2 before being adjusted by the voltage adjusting circuit 215, and is smaller than the reference voltage VREF2 after being adjusted by the voltage adjusting circuit 215. More specifically, when the amplified feedback voltage VFBO is lower than the control voltage VC, which means the output voltage VOUT is too low, and the switch Q1 is needed to be turned on for allowing the input voltage source VIN to charge the inductor L and the output capacitor COUT, the comparator CMP2 generates the comparing signal SC to trigger the pulse generator 211 to generate the pulse signal PON. When the pulse generator 211 is triggered by the comparator CMP1, the pulse generator 211 generates a pulse signal PON with a predetermined duration TP and predetermined logic. The drive circuit 212 controls the switch set 220 according to the pulse signal PON. More particularly, when the drive circuit 212 receives the pulse signal PON, the switch Q1 is driven to turn on for the predetermined duration TP and the input voltage source VIN conducts through the turned-on switch Q1 to charge the inductor L and the output capacitor COUT for raising the output voltage VOUT. Meanwhile, the pulse signal PON controls the switch SW to turn on for conducting the constant current source IS for lowering the control voltage VC. After the predetermined duration TP of the pulse signal PON, the switch SW is turned off so that the constant current source IS cannot discharge the resistor RX and the capacitor CX. Instead, the voltage reference source VREF2 charges the resistor RX and the capacitor CX for raising the control voltage VC. By repeatedly executing the above actions, the control voltage VC can be of the saw-toothed waveform.
In other words, the inductor L starts to be charged at the beginning of the pulse signal PON and the current IL increases, and the inductor L starts to be discharged at the end of the pulse signal PON and the current IL decreases. Simply speaking, in the present invention, the pulse signal PON is used to simulate the variation of the current IL on the inductor L and is fed back to the comparing circuit 213 to correspondingly adjust the control voltage VC, i.e. the control voltage VC varies in response to the variation of the current IL. In this way, the control circuit 210 is capable of stably controlling the DC/DC converter 200 without the equivalent serial resistor RE.
To sum up, the control circuit of the present invention, by utilizing the pulse signal to simulate the change of the current on the inductor, is still capable of stably controlling the bulk DC/DC converter when there is no equivalent serial resistor, providing great convenience.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

## Claims

1. A control circuit in constant on-time mode for controlling a DC/DC converter, comprising: a comparing circuit comprising: a comparator comprising: a positive input end for receiving a control voltage; a negative input end for receiving a feedback voltage divided from an output voltage of the DC/DC converter; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; wherein the voltage adjusting circuit comprises: a first resistor coupled to the positive input end of the comparator; a first capacitor coupled to the positive input end of the comparator; a first voltage reference source coupled to the first resistor, the first capacitor, and ground; a first switch, comprising: a first end coupled to the positive input end of the comparator; a second end; and a control end coupled to the pulse generator for receiving the pulse signal; and a constant current source coupled to the second end of the first switch; and
a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal for controlling a switch set of the DC/DC converter; the pulse generator coupled to the control end the first switch for transmitting the pulse signal to the control end of the first switch wherein the first end of the first switch conducts the second end of the first switch for allowing the constant current source to discharge the first resistor and the first capacitor in order to lower when the first switch receives the pulse signal.

2. The control circuit of claim 1, wherein the comparator outputs the comparing signal when the control voltage is higher than the feedback voltage.

3. The control circuit of claim 1, wherein duration of the pulse signal is fixed.

4. The control circuit of claim 1, further comprising a drive circuit coupled between the pulse generator and the switch set of the DC/DC converter for driving the switch set of the DC/DC converter according to the pulse signal.

5. The control circuit of claim 1, further comprising an amplifying circuit, comprising:
an amplifier, comprising:
a positive input end for receiving the feedback voltage;
a negative input end coupled to a second voltage reference source for receiving a second reference voltage; and
an output end coupled to the negative input end of the comparator for outputting the amplified feedback voltage;

a second resistor coupled to the positive input end of the comparator; and
a second capacitor coupled between the second resistor and the ground.

6. A bulk DC/DC converter in constant on-time mode, comprising: a switch set coupled between an input voltage source and ground; an inductor coupled to the switch set for receiving the input voltage source through the switch set; an output end coupled to the inductor for outputting an output voltage; an output capacitor coupled between the capacitor and the ground; a voltage-dividing set coupled between the output end and the ground for generating a feedback voltage according to the output voltage; and a control circuit, comprising: a comparing circuit comprising: a comparator comprising: a positive input end for receiving a control voltage; a negative input end coupled to the voltage-dividing set for receiving the feedback voltage; and an output end for outputting a comparing signal; and a voltage adjusting circuit coupled to the positive input end of the comparator for adjusting the control voltage; wherein the voltage adjusting circuit comprises: a first resistor coupled to the positive input end of the comparator; a first capacitor coupled to the positive input end of the comparator; a first voltage reference source coupled to the first resistor, the first capacitor, and the ground; a first switch, comprising: a first end coupled to the positive input end of the comparator; a second end; and a control end coupled to the pulse generator for receiving the pulse signal; and a constant current source coupled to the second end of the first switch;
a pulse generator coupled to the output end of the comparator for generating a pulse signal according to the comparing signal so as to transmit the input voltage source to the inductor through the switch set; the pulse generator coupled to the control end the first switch for transmitting the pulse signal to the control end of the first switch wherein the first end of the first switch conducts the second end of the first switch for allowing the constant current source to discharge the first resistor and the first capacitor in order to lower when the first switch receives the pulse signal;
a drive circuit coupled between the pulse generator and the switch set of the DC/DC converter for driving the switch set of the DC/DC converter according to the pulse signal; and an amplifying circuit, comprising: an amplifier, comprising: a positive input end for receiving the feedback voltage; a negative input end coupled to a second voltage reference source for receiving a second reference voltage; and an output end coupled to the negative input end of the comparator for outputting the amplified feedback voltage; a second resistor coupled to the positive input end of the comparator; and a second capacitor coupled between the second resistor and the ground.

7. The bulk DC/DC converter of claim 6, wherein the comparator outputs the comparing signal when the control voltage is higher than the feedback voltage.

8. The bulk DC/DC converter of claim 6, wherein duration of the pulse signal is fixed.

9. The bulk DC/DC converter of claim 6, wherein the switch set comprises:
a third switch coupled between the input voltage source and the inductor; and
a second switch coupled between the ground and the inductor;
wherein when the pulse signal is generated, the third switch is turned on for transmitting the input voltage source to the inductor.

10. The bulk DC/DC converter of claim 6, wherein the output capacitor is a multi-layer ceramic capacitor (MLCC).